(1) Field of the Invention
The present invention relates to a liquid crystal display device and a manufacturing method therefor. More particularly, it relates to a liquid crystal display device and a manufacturing method therefor, wherein a plurality of liquid crystal layers are stacked on a substrate to achieve bright, full-color display of an image.
(2) Description of the Related Art
A typical liquid crystal display device is composed of a liquid crystal sealed between two glass substrates joined together with a specified spacing maintained therebetween.
There have been used liquid crystals in various modes in accordance with the orientation of liquid crystal molecules. Among them are: a twisted nematic liquid crystal (hereinafter referred to as TN liquid crystal), which is the most prevalent liquid crystal; a birefringence liquid crystal in homeotropic (perpendicular) alignment or in homogeneous (parallel) alignment; and a guest-host liquid crystal.
A monochrome liquid crystal display device using the TN liquid crystal is typically constituted such that a TN liquid crystal having positive dielectric anisotropy is sealed between a pair of substrates that have undergone parallel orientation treatment and are formed with pixel electrodes and a counter electrode. The pair of glass substrates are disposed between a polarizer and an analyzer placed with their respective planes of polarization orthogonal to each other.
The monochrome liquid crystal display device as mentioned above achieves display based on the following principle. When no voltage is applied between the electrodes, the alignment of the TN liquid crystal is parallel to the glass substrates with a stable twist of 90.degree.. While passing through the TN liquid crystal, light incident through the polarizer has its plane of polarization rotated 90.degree. in accordance with the twisted alignment of the TN liquid crystal, so that it is allowed to pass through the analyzer. When a voltage is applied between the electrodes, on the other hand, the alignment of the TN liquid crystal is perpendicular to the glass substrates. Accordingly, the light incident through the polarizer passes through the TN liquid crystal with its plane of polarization unrotated but cannot pass through the analyzer since it is absorbed therein. The transmission or blocking of the light Is controlled by the presence or absence of the applied voltage between the electrodes, which provides white or black display.
A color liquid crystal display device using the TN liquid crystal is similarly constituted to the monochrome liquid crystal display device, except that a higher-definition liquid crystal panel and a micro color filter having red, green, and blue regions corresponding to each set of three adjacent pixel electrodes are combined to compose the color liquid crystal display device. Such a color liquid crystal display device achieves full-color display by additive color mixing.
However, the color liquid crystal display device using the TN liquid crystal has difficulty in providing sufficiently high brightness. This is because the micro color filter has a low light transmittance and only the component of incident light having a plane of polarization coincident with that of the polarizer is allowed to pass through the polarizer. Accordingly, the overall transmittance obtained is 10% or lower.
The liquid crystal display device using the birefringence liquid crystal also has a similar problem resulting from the polarizer provided therein.
These problems are particularly remarkable when the foregoing constitution is applied to a reflection-type liquid crystal display device utilizing external light source. In that case, brightness is lowered to a point where color is barely recognizable.
To achieve bright color display, a liquid crystal display device using a guest-host liquid crystal having dichroic dyes is disclosed in Japanese Unexamined Patent Publication SHO 61-238024 or HEI 3-238424.
The liquid crystal display device using the guest-host liquid crystal has a plurality of liquid crystal panels stacked in layers, each composed of a liquid crystal containing a dichroic dye of a different color sealed between a pair of glass-substrate. The glass substrates are normally formed with transparent pixel electrodes and a transparent counter electrode, each composed of indium tin oxide. More specifically, the liquid crystal display device is composed of three liquid crystal panels having therein respective liquid crystals containing yellow, magenta, and cyan dyes to selectively absorb blue, green, and red light depending on the voltage applied. If each of the liquid crystal panels absorbs light of the corresponding color, the liquid crystal display device displays black. If some of the liquid crystal panels absorb light of the corresponding colors, the liquid crystal display device achieves color display. If none of the liquid panels absorbs light, the liquid crystal display device displays white. In short, the liquid crystal display achieves full-color display by subtractive color mixing. Since the liquid crystal display device does not comprise a micro color filter nor polarizer absorbing a large amount of light, it can display relatively bright images compared with the color liquid crystal display device using the TN liquid crystal.
However, even the conventional liquid crystal display device using the guest-host liquid crystal cannot display by far brighter images, whether it is of transmission type having a backlight or of reflection type having a reflector plate. This is because, in the liquid crystal display device of transmission type, light emitted from the backlight should pass through the six glass substrates and the six transparent electrodes. In the liquid crystal display device of reflection type, the incident light through the front plate and the reflected light from the reflector plate should pass through the total of twelve glass substrates and the total of twelve transparent electrodes.
On the other hand, the high-definition liquid crystal display device having a pixel pitch much smaller than the thickness (normally on the order of 1 mm) of the glass substrate has a serious drawback of color displacement experienced when viewed obliquely under the great influence of parallax. To reduce the parallax, a plastic substrate, which can be reduced in thickness more easily than the glass substrate, may be used instead of the glass substrate. However, if consideration is given to the handing of the plastic substrate in the manufacturing process including a lamination step, even a film-like plastic substrate needs a minimum thickness of 0.05 mm or more, normally a thickness of about 0.1 to 0.3 mm, so that it is still difficult to eliminate the influence of parallax.
Since the liquid crystal display device of this type is composed basically of a lamination of three liquid crystal panels, it is necessary to repeat three times the step of joining a pair of glass substrates together and injecting a liquid crystal into the space therebetween, which corresponds to approximately triple the process of manufacturing the liquid crystal display device using the TN liquid crystal, resulting in higher manufacturing cost.
To eliminate the color displacement due to parallax and reduce the manufacturing cost, Japanese Unexamined Patent Publication HEI 6-337643 has proposed a liquid crystal display device of so-called polymer dispersed type, which is shown in FIG. 20. The polymer dispersed liquid crystal display device is composed of liquid crystal layers 295 to 297 and transparent pixel electrodes 292 to 294 stacked on a single glass substrate 291 to form a multi-level structure. In the liquid crystal layer 295, 296, or 297, tiny droplets of a liquid crystal 299 are dispersed and contained in a polymer 298 that has been solidified.
In the liquid crystal display device, since the liquid crystal 299 is held in the solidified polymer 298, it is unnecessary to prepare a glass substrate for each of the liquid crystal layers 295 to 297, which eliminates the color displacement resulting from parallax and reduces the size and weight of the liquid crystal display device. Moreover, each of the liquid crystal layers 255 to 297 can be formed by applying the fluid polymer 298 onto the glass substrate 291 or onto another liquid crystal layer 295 or 296 with a roll coater or spinner and sintering the applied polymer 298 such that it is solidified, resulting in simpler manufacturing process.
However, the polymer dispersed liquid crystal display device has the following drawback.
In recent years, a majority of liquid crystal display devices have adopted an active matrix addressing system wherein switching elements composed of TFTs (Thin Film Transistors) are provided for the respective transparent pixel electrodes of the individual liquid crystal layers to control switching between the application and no application of a voltage to the transparent pixel electrodes. The active matrix addressing system is for increasing a display response speed up to a point where dynamic images can be displayed. When the active matrix addressing system is applied to the foregoing liquid crystal display device, however, switching elements corresponding to transparent pixel electrodes 293 and 294 of the second and third liquid crystal layers 296 and 297 are formed on a glass substrate 291 on which the first liquid crystal layer 295 has been formed since the second and third liquid crystal layers 296 and 297 do not have their own glass substrates, as described above. In addition, it is also necessary to provide multi-level interconnections between the switching elements and the transparent pixel electrodes 234 and 294.
In view of the foregoing, the liquid crystal display device proposed in the foregoing publication has used a negative photosensitive polymer as the polymer 298 which is cured on exposure to UV light. Specifically, the polymer 298 applied to the glass substrate 291 is cured on exposure to UV light except for portions located in the vicinities of the switching elements, while the unexposed portions are removed in a developing agent or the like, resulting in openings. The transparent pixel electrodes 293 and 294 are connected to the switching elements via the openings. However, the exposure of the polymer 298 containing the liquid crystal 299 to UV light causes the degradation of the liquid crystal 299 (particularly the dye contained therein).
As disclosed in Japanese Patent Publication HEI. 8-146456, the present inventors have proposed a liquid crystal display device wherein a transparent pixel electrode can easily be connected to a switching element, which is fabricated as follows. The portions of a glass substrate corresponding to individual pixels are subjected to etching using a solution containing a hydrofluoric acid as the main component to form concave portions. To the concave portions, a polymer containing a liquid crystal is transferred by printing or like technique, similarly to the foregoing liquid crystal display device, resulting in a liquid crystal layer. This reduces the distance between the transparent pixel electrode formed on the liquid crystal layer and the surface of the unetched portion of the glass substrate, so that the transparent electrode is easily connected to the switching elements. However, the liquid crystal display device is disadvantageous in that the step of forming the concave portions requires considerable labor.
In the foregoing liquid crystal display device with the liquid crystal layers 295, 296, and 297 formed from the polymer 298 applied onto the glass substrate 291, a pinhole is easily formed in the applied polymer 298 due to uneven coating or residual bubbles, while projecting or depressed portions are frequently observed on the surfaces of the liquid crystal layers 295, 296, and 297 because of the droplets of the liquid crystal 299 broken and flowing out due to a difference in thermal expansion coefficient between the polymer 298 and the liquid crystal 299. The pinhole causes a short circuit between the transparent pixel electrodes 292 to 294, while the projecting or depressed portions cause discontinuities (disconnections) in the transparent pixel electrodes 292 to 294. Even when no pinhole is observed, if the uneven coating of the polymer 298 has an extremely thin portion, a short circuit is caused by a dielectric breakdown when a voltage is applied to the transparent electrodes 292 to 294. In any case, normal display is prohibited.
Moreover, the walls of the polymer 298 containing the droplets of the liquid crystal 299 should be formed sufficiently thick (0.3 to 0.5 .mu.m) not to be broken easily. Accordingly, the portion without the liquid crystal accounts for an area ratio of about 30% to 60%, which is considerably high. As a result, the effective aperture ratio is reduced and satisfactorily high contrast cannot be obtained.
The present inventors have also proposed another liquid crystal display device with a view to eliminating the color displacement resulting from parallax and providing improved contrast, which is fabricated as follows. A resist patterned into a specified configuration is formed on a glass substrate, followed by an insulating film and a transparent pixel electrode formed thereon. The resist is then exposed to UV light and removed in a developing agent so as to form a gap between the glass substrate and the insulating film, into which a guest-host liquid crystal is injected. Similarly to the foregoing gap, openings for providing connection between the transparent pixel electrode and a switching element on the glass substrate, such as those described above in the polymer dispersed liquid crystal display device, are also formed easily.
Since the liquid crystal display device comprises, in place of a thick glass substrate, the insulating film that can be formed extremely thin to seal the liquid crystal, the color displacement resulting from parallax is eliminated. In addition, since the gap is filled with the liquid crystal and does not contain a foreign substance (a polymer resin as a dispersed phase), the contrast can easily be increased to a certain extent. However, the liquid crystal display device has the following problem to be solved.
Since the resist dissolved in the developing agent is blocked by the insulating film and less likely to be diffused, the resist is not equally removed when an inlet for the developing agent is small, which may lower the production yield. If the inlet for the developing agent is enlarged, on the other hand, it becomes difficult to provide an aperture ratio greatly higher than the aperture ratio of the foregoing polymer dispersed liquid crystal display device.
The following are problems common to some of the foregoing conventional liquid crystal display devices, which concern a reduction in display performance.
A typical liquid crystal display device having a liquid crystal layer composed of a liquid crystal filled in a space between a pair of glass substrates or sealing films employs spacers to maintain the liquid crystal layer at a given thickness, exclusive of the polymer dispersed liquid crystal display device. In the case of using transparent spacers, they form luminescent spots, which are inevitably displayed on a screen. In the case of using black spacers, they form black spots, which are inevitably displayed on the screen. Since each layer has spacers in a liquid crystal display device of tri-layered structure, the area occupied by the spacers is large relative to the entire area of the screen. Consequently, the spacers exert a great influence on the conventional liquid crystal display device of tri-layered structure.
In a reflection-type color liquid crystal display device having three guest-host liquid crystal layers of cyan, magenta, and yellow as well as a reflector film disposed on the back surface, even when the liquid crystal layers are stacked by measuring the respective absorption spectra of the individual colors and determining the dye concentrations and thicknesses of the liquid crystal layers such that monochrome display is achieved in the ON or OFF state, both white and black are slightly tinted with a color, especially with the color of the foreground liquid crystal layer.